1. Field of the Invention
The present invention relates to a mask frame assembly and an organic light emitting display (OLED) device manufactured using the mask frame assembly, and more particularly, to a mask frame assembly for depositing a thin layer and an OLED device manufactured using the mask frame assembly.
2. Discussion of the Background
Generally, self-emissive OLED display devices are expected to be next generation display devices since they may have a wide viewing angle, high contrast, and high response speed.
Display devices may be inorganic light emitting display devices and OLED devices according to the material that forms the light emitting layer. The OLED devices are typically brighter than inorganic devices, and they have a higher response speed. Additionally, they can display color images.
The OLED device comprises a first electrode formed in a predetermined pattern on a substrate, an organic light emitting layer, which may be formed by vacuum evaporation, on the substrate having the first electrode, and a second electrode (cathode) formed in a direction crossing the first electrode on the organic light emitting layer.
The first electrode may be formed of indium tin oxide (ITO), and the ITO may be patterned using a photolithography method. In other words, the ITO may be patterned by wet etching with an etching solution including ferric chloride FeCl3. However, if the second electrode is also etched using photolithography, moisture may penetrate a surface between the organic light emitting layer and the second electrode when exfoliating a resist and etching the second electrode, which reduces the OLED's performance and lifetime.
A method of depositing materials for forming an organic light emitting layer and a second electrode has been suggested to solve this problem.
To manufacture an OLED device using this deposition method, a stripe-shape first electrode formed of ITO is formed on a transparent insulating substrate using a photolithography method. After stacking an organic light emitting layer on the substrate having the first electrode, a mask having the same pattern as the second electrode is arranged contacting the organic light emitting layer, and a second electrode is formed by depositing a material for forming the second electrode.
Korea Laid-Open Patent Publication 2000-060589 discloses a mask for depositing the organic light emitting layer or the second electrode, an OLED device that uses the mask, and a method of manufacturing the OLED device.
The mask disclosed in Korea Laid-Open Patent Publication 2000-060589 has slots in a stripe shape spaced a predetermined distance from each other on a main body of a thin film.
A mask disclosed in Korea Laid-Open Patent Publication 1998-0071583 has a mesh shaped slit part and a bridge part on a metal thin film.
A mask disclosed in Japanese Patent laid-open Publication No 2000-12238 includes an electrode mask part and a pair of terminal mask units. The electrode mask part has a width corresponding to a gap between second electrode (i.e. cathodes), a plurality of marking parts in a stripe shape disposed parallel to each other, and a connection part that connects both ends of each marking part.
The conventional masks described above may not closely adhere to the substrate due to their weight even though a frame of the mask is tensioned to support an edge of a metal thin film in which long, stripe-shaped holes are formed. This problem is more severe as substrate size increases. Also, when the cathode is deposited, sagging due to the weight of the slot strips increases since heat applied to the mask causes it to expand.
FIG. 1 is an exploded perspective view showing a conventional mask used for mass production.
Referring to FIG. 1, masking pattern units 12 for depositing a plurality of unit substrates that form an OLED device are included on a metal thin film 11, and the mask is fixed to a frame 20 while applying tension to the mask.
The sagging problem of the conventional mask 10 may be more severe even though tension is uniformly applied to the frame 20 formed in a lattice shape since the mask 10 is large. In particular, a large metal thin film mask should be welded to the frame 20 so that a width of slots 12a formed on each of the masking pattern units 12 may be maintained within a tolerance range. If tension is applied to prevent the mask 10 from sagging, the welding within a tolerance range is difficult since a slot 12a pitch of each of the masking pattern units 12 may be deformed. If the slot 12a of the masking pattern units 12 of a particular portion of the mask 10 is deformed, a deformation force is applied to all neighboring slots. Therefore, the designed pattern deviates from a tolerance range since the slots 12a move relative to the substrate to be deposited. This problem arises in a vertical direction to the length direction of the slots 12a formed on the mask.
When each of the masking pattern units 12 is deformed, an accumulated deformation corresponding to absolute location errors between electrode pattern units formed on a substrate and the masking pattern units 12 increases. When the accumulated deformation increases, correct organic films of red, green, and blue color may not be formed on the electrode pattern units of the substrate. Hence, there is a limit to increase the size of the mask 10 since the control of pitch and total pitch of the masking pattern units 12 formed on a large metal thin film may be possible only in a very limited portion.
As FIG. 2 shows, when the mask 10 for one OLED device is fixed on the frame 20 by applying tension, the supporting bars 21 of both sides of the frame 20 may curve inward of the mask 10 and the upper and lower supporting bars 22 may bow outward, or, as FIG. 3 shows, both side supporting bars 21 may bow outward and the upper and lower supporting bars 22 may curve inward.
The deformations make it more difficult to control a total pitch difference between the masking pattern units 12 and the electrode pattern units formed on the substrate even if the mask 10 is welded to the frame 20 by applying uniform tension to the mask 10.
Japanese Patent Laid-Open publication 2001-247961 discloses a mask for solving a problem of creep deformation of strips that form slots due to thermal expansion of the mask. The disclosed mask is a deposition mask used for forming a patterning film by deposition on a substrate and includes a mask part having a plurality of barrier ribs that define first openings and a screen part in which a plurality of second openings, which have a smaller opening area than each first opening, include magnetic materials disposed on each of the first openings of the mask unit.
Japanese Patent Laid-Open publication 2001-273979 discloses a structure of a magnetic mask, and Japanese Patent Laid-Open publication 2001-254169 discloses a structure of a mask frame for masking a deposition region by adhering to a material to be deposited. The mask frame, on which a mask pattern corresponding to a deposition region is formed, includes a mask pattern having fine patterning units, which are supported by fine ribs.
These masks, however, do not solve the problems of sagging due to the mask's weight, of deformation of pitch between the strips due to tension applied to the mask, and of variation of total pitch caused by internal stress of the mask and the frame even though the mask is closely adhered to a material to be deposited by forming a magnetic body.
Additionally, Japanese Patent Laid-Open publication 2002-235165 and U.S. Pat. No. 3,241,519 disclose masks for preventing thermal deformation of openings and improving precision. A mask in which a plurality of unit patterns are formed on a single frame and mask for a large display pattern has been disclosed in European Patent Laid-Open Publication EP 1,209,522 A2, and similar masks have been disclosed in U.S. Published Application No. 2002/0025406 A1, now U.S. Pat. No. 6,898,709. However, these masks also have the same problems described above.
Also, European Patent Laid-Open Publication EP 1,229,144 A2 discloses a mask frame assembly in which a plurality of masks are supported by a single frame having openings corresponding to each of the masks. However, this mask may waste substrate to be deposited due to the limitation of reducing gaps between masks, and the fabrication of masks is complicated. Therefore, this mask may not be suitable for forming a large size display pattern.
Applicant's Korean Patent Publication 2003-0046090 discloses a mask frame assembly to solve the above problems.
The mask includes at least two unit masks on which at least one masking pattern unit is formed in a length direction of the mask. However, a gap between the unit masks can be hardly reduced.